1. Field of the Invention
The general field of the present invention is that of mobile radio systems.
The more particular field of the present invention is that of packet mode services, such as the General Packet Radio Service (GPRS), especially for Global System for Mobile communications (GSM) mobile radio systems.
2. Description of the Prior Art
Generally speaking, the above systems are covered by standards, and for more information reference may be made to the corresponding standards published by the corresponding standardization organizations.
The architecture of packet mode systems, such as GPRS systems, for example, is outlined in FIG. 1, and essentially comprises:                a base station subsystem (BSS) communicating with mobile stations (MS) and including base transceiver stations (BTS) and base station controllers (BSC),        a GPRS network subsystem connected to the BSS and to external networks (not shown), and including GPRS network subsystem entities or nodes, such as serving GPRS support nodes (SGSN) and gateway GPRS support nodes (GGSN).        
In accordance with the multilayered architecture used to define the above systems, the interface between the MS and the BSS, also known as the radio interface or the “Um” interface, includes:                a first layer, also known as the physical layer, and        a second layer, also known as the link layer, which is divided into a plurality of layers, as follows, in increasing level order: a medium access control (MAC) layer, a radio link control (RLC) layer, and a logical link control (LLC) layer, the BSS providing only a relay function between MS and the GPRS network subsystem for the LLC layer.        
Similarly, the interface between the BSS and the GPRS network subsystem, also known as the “Gb” interface, includes:                a first layer, also known as the physical layer, and        a second layer, also known as the link layer, which is divided into a plurality of layers, as follows, in increasing level order: a network service layer, a BSS GPRS protocol (BSSGP) layer, and a logical link control (LLC) layer, the BSS providing only a relay function between MS and the GPRS network subsystem for the LLC layer.        
Frames referred to as LLC frames are formed in the LLC layer from higher level data units which are referred to in the LLC frames as LLC-protocol data units (LLC-PDU).
The LLC-PDUs are then segmented in the RLC/MAC layer to form blocks known as RLC data blocks. The RLC data blocks are then converted to the format required for transmission at the “Um” interface in the physical layer.
Furthermore, procedures are implemented in the RLC and LLC layers for forwarding data (RLC data blocks or LLC-PDU, as appropriate) that has not been received correctly, in accordance with the automatic repeat request (ARQ) technique. The receiver signals the correct or incorrect state of the data blocks or units received to the transmitter by means of acknowledgement (ACK) messages and non-acknowledgement (NACK) messages.
Furthermore, higher level signaling protocols are also provided, in particular for mobility management (MM), session management (SM), etc.
The following description outlines some procedures relating to the RLC/MAC protocol at the interface between the MS and the BSS. For a more complete description of this protocol, see Technical Specification (TS) 04.60 published by the 3rd Generation Partnership Project (3GPP).
In packet mode the logical channels comprise the following channels:                packet broadcast control channels (PBCCH), used to transmit system information in a cell,        packet common control channels (PCCCH), in turn comprising the following channels:                    a packet random access channel (PRACH), used to access the network,            a packet paging channel (PPCH), used to page users,            a packet access grant channel (PAGCH), used to allocated resources in packet mode,            a packet notification channel (PNCH), used to notify mobile stations of a point-to-multipoint call, and                        packet data transfer channels (PDTCH) used to transfer data and packet associated control channels (PACCH) used in particular to transmit acknowledgements (ACK/NACK) or messages connected with allocation/modification of resources in packet mode.        
Some packet mode channels, such as the PBCCH and PCCCH in particular, cannot be established in a cell. In this case, the mobile stations in packet mode use circuit mode channels such as the broadcast control channel (BCCH) and the packet common control channel (CCCH), the latter including in particular the random access channel (RACH), the paging channel (PCH), the access grant channel (AGCH), and the notification channel (NCH). To indicate this facility, the notation (P)BCCH is used to refer to the PBCCH and to the BCCH, for example.
In packet mode, a mobile station can be:                either in a packet transfer mode, in which resources are allocated temporarily when there is actually data to be transmitted during a call, the resources allocated temporarily forming a temporary packet mode connection or temporary block flow (TBF) for a given transmission direction,        or in a packet idle mode in which no TBF is established.        
Generally speaking, data transfer by means of a TBF can be data referred to herein as user data or data referred to herein as signaling data, exchanged in the context of higher level protocols, for example the mobility management (MM) protocol, etc.
An uplink TBF (UL TBF) can be established either on the common control channels (P)CCCH or on the PACCH of a downlink TBF (DL TBF) simultaneously operative for the mobile station concerned.
The establishing of a UL TBF on the common control channels (P)CCCH is initiated by the mobile station sending the network a PACKET CHANNEL REQUEST message on the PRACH (or a CHANNEL REQUEST message on the RACH). Depending on what the mobile station requires for the transfer in question, different causes can be used at the time of a request to establish a UL TBF: one phase access, short access, two phase access, cell update, page response, MM procedure, single block without TBF establishment.
The following steps apply in the case of one phase access or two phase access, for example:                In the case of one phase access, the network responds with an IMMEDIATE ASSIGNMENT message (respectively a PACKET UPLINK ASSIGNMENT message) on the AGCH (respectively the PAGCH), this message indicating directly to the mobile station the packet resources or PDCHs allocated.        In the case of two phase access, the IMMEDIATE ASSIGNMENT (PACKET UPLINK ASSIGNMENT) message on the (P)AGCH allocates the mobile station a radio block for a PDCH, which it can use to transmit a PACKET RESOURCE REQUEST message containing a more precise description of the required packet mode resources. The network then responds with a PACKET UPLINK ASSIGNMENT message indicating to the mobile station the packet mode resources or PDCHs allocated.        
In the case of establishing an uplink TBF (UL TBF) on the PACCH of a downlink TBF (DL TBF) operative simultaneously for the mobile station concerned, the network sends the PACKET UPLINK ASSIGNMENT message on the PACCH.
A downlink TBF (DL TBF) can be established either on the common control channels (P)CCCH or on the PACCH of an uplink TBF (UL TBF) operative simultaneously for the mobile station concerned.
In the case of establishing a DL TBF on the common control channels, if the SGSN does not know the cell in which the mobile station is located, it can first initiate a paging procedure in packet mode via the BSCs likely to be controlling that cell. An exchange of signaling on the (P)CCCH then follows, including the mobile station sending a CHANNEL REQUEST message (or PACKET CHANNEL REQUEST message on the PRACH) in response to the paging, followed by the network sending the mobile station a PACKET UPLINK ASSIGNMENT message indicating to the mobile station the packet mode resources or PDCHs allocated, on which the mobile station can send its response to the SGSN. Consequently, having determined the cell in which the mobile station is located, the SGSN can send LLC data units to the corresponding BSC, which then establishes a DL TBF either on the (P)CCCH or on the PACCH of the UL TBF if it is still operative. To establish a DL TBF on a common channel, the BSC sends an IMMEDIATE ASSIGNMENT message on the CCCH or a PACKET DOWNLINK ASSIGNMENT message on the PCCCH, the message indicating to the mobile station the packet mode resources or PDCHs allocated.
In the case of establishing a DL TBF on the PACCH of a UL TBF operative simultaneously for the mobile station concerned, the network sends the PACKET DOWNLINK ASSIGNMENT message on the PACCH.
Moreover, the above systems have a cellular architecture, and handover mechanisms are provided. The following description outlines some procedures relating to these handover mechanisms. For a more complete description of these mechanisms see Technical Specifications (TS) 04.60 and 05.08 published by the 3rd Generation Partnership Project (3GPP).
A procedure known as cell reselection is generally used for packet mode services, and a distinction is generally drawn between a number of cell reselection control modes, corresponding to decreasing degrees of autonomy of the mobile station or, which amounts to the same thing, increasing degrees of control by the network. In the case of the GPRS, for example, the control modes include:                a first control mode NC0 in which the mobile station decides autonomously to effect handover and itself selects the target cell, taking account of the results of measurements that it carries out,        a second control mode NC1 in which the mobile station decides autonomously to effect handover and itself selects the target cell, taking account of the results of measurements that it carries out, and also transmits the results of the measurements to the network, and        a third control mode NC2 in which the network decides to effect handover and selects the target cell, taking account of measurement results that the mobile station sends it.        
Thus the control modes NC0 and NC1 correspond to a mode of cell reselection controlled by the mobile station. In this case, the mobile station itself decides to effect a cell reselection.
The control mode NC2 is also known as cell reselection controlled by the network. In this case, the network instructs the mobile station to effect a cell reselection in a PACKET CELL CHANGE ORDER message containing the identity of the reselected cell.
In any of the control modes, for example NC0, NC1, NC2, once the mobile station has successfully effected the operations necessary to connect it in packet mode to the target reselected cell, it sends the network a cell update message indicating the identity of the target reselected cell using the mobility management (MM) protocol. If the mobile station has no user data to send, it sends the SGSN an empty LLC PDU and uses the “cell update” cause at the time of the request to establish a UL TBF for sending said LLC PDU when the PBCCH is established. If the PBCCH is not established in the cell, the mobile station requests one phase establishment. Alternatively, if the mobile station has user data to transfer in the uplink direction, it must send a packet mode resource request message to request the network to establish a UL TBF. Once the UL TBF has been established, the mobile station must send its user data, which is also interpreted by the SGSN as a cell update. On detecting the cell update, the SGSN can then resume, to the new cell, the transfer of data to the old cell that was interrupted.
Moreover, the GPRS standards have evolved, in particular with the introduction of the Enhanced General Packet Radio Service (EGPRS), which offers bit rates very much higher than those offered by the GPRS, thanks to modulation techniques that are more spectrum efficient.
Nevertheless, not all the mobile stations and all the cells in the same system necessarily support the EGPRS. The following description outlines some procedures enabling the GPRS and the EGPRS to coexist in the same system. For more complete description of these procedures reference may be made to the Technical Specification (TS) 04.60 published by the 3rd Generation Partnership Project (3GPP).
There are two modes for a TBF, namely a GPRS mode and an EGPRS mode. Moreover, if a mobile station has a UL TBF and a DL TBF operative simultaneously, then the two TBF must be in the same mode, either the GPRS mode or the EGPRS mode.
Moreover, as the packet mode resource request messages previously referred to, namely the PACKET CHANNEL REQUEST message and the CHANNEL REQUEST message, do not themselves indicate if the mobile station supports the EGPRS, a new packet mode resource request message, namely the EGPRS PACKET CHANNEL REQUEST message, has been introduced.
Not all the cells necessarily support the EGPRS PACKET CHANNEL REQUEST message, and support for this message in a cell is indicated in system information broadcast in the cell on the (P)BCCH.
The EGPRS PACKET CHANNEL REQUEST message can be sent on the (P)CCCH. A mobile station sending this message in itself indicates that the mobile station supports the EGPRS. The only way for the network to know if a mobile station supports the EGPRS at the time of establishing a UL TBF on the (P)CCCH is to receive the EGPRS PACKET CHANNEL REQUEST message. The network can then establish a UL TBF in the EGPRS mode. If not, the network can only establish a TBF in the GPRS mode.
According to the current version of the standard, the packet mode resource request messages are used in the following circumstances:                if the cell supports the EGPRS PACKET CHANNEL REQUEST message:                    if the mobile station requires to effect a one phase access, a two phase access, or a short access, it uses the EGPRS PACKET CHANNEL REQUEST message (with the appropriate cause),            if the mobile station wishes to effect a cell update, to send a packet mode page response, to execute a mobility management (MM) procedure, or to request the allocation of a single block without TBF establishment, it uses the CHANNEL REQUEST message (if the PBCCH is not present in the cell) or the PACKET CHANNEL REQUEST message (if the PBCCH is present in the cell),                        if the cell does not support the EGPRS PACKET CHANNEL REQUEST message the mobile station uses the CHANNEL REQUEST message or the PACKET CHANNEL REQUEST message in all circumstances.        
The EGPRS is especially beneficial for applications such as Internet access in particular. In this kind of application, the user data transferred is exchanged in accordance with the Transmission Control Protocol (TCP) which is itself defined in accordance with the Transmission Control Protocol/Internet Protocol (TCP/IP) model. A typical situation in this kind of application corresponds to a downlink TBF established in the EGPRS mode for transferring user data and an uplink TBF established from time to time, also in the EGPRS mode, for transmitting TCP acknowledgements (TCP ACKs).
In this kind of application in particular, the current version of the standard gives rise to problems that we have recognized and are explained next.
The examples shown in FIGS. 2 and 3 consider an initial state, which is denoted 1 in FIG. 2 and 1′ in FIG. 3, and corresponds to a situation of this kind in which a transfer of data is in progress between an equipment corresponding to a mobile station (MS) and an equipment corresponding to a BSS, in this example the BSS of an old cell, prior to handover. TCP segments, or data units exchanged in accordance with the TCP, are transmitted in the downlink direction, and these TCP segments are identified by a sequence number and TCP acknowledgements (TCP ACKs) and are then transmitted in the uplink direction, the acknowledgements being identified by their ACK number.
Consider next the handover situation, in which either the MS has decided on cell reselection (mode NC0 or NC1) or the network has instructed the MS to effect cell reselection (mode NC2). This situation is also one in which the MS has successfully effected the operations necessary to connect to the new (reselected) cell (corresponding to the BSS(new cell) equipment). The corresponding state is denoted 2 in FIG. 2 and 2′ in FIG. 3. Consider further the situation of a new (or reselected) cell which supports the PBCCH and the EGPRS PACKET CHANNEL REQUEST message (note, however, that the scenarios explained apply equally well to the situation in which there is no PBCCH in the cell).
In the current version of the standard, there are two scenarios available for resuming, to the new cell, the transfer to the old cell that was interrupted.
A first scenario corresponds to the situation in which the MS still has one or more LCC PDU(s) to send to the network (corresponding to TCP ACKs that were not sent in the old cell).
This first scenario corresponds to the example shown in FIG. 2. In that example, before changing to state 2, TCP segments with sequence numbers “n” and “n+1” have been transmitted in the downlink direction and a TCP ACK having the number “n+1” has been transmitted in the uplink direction, while the TCP ACK having the number “n+2” has not yet been transmitted.
In this situation, to transmit to the network the TCP ACK having the number “n+2”, the MS requests a short access (or a one phase access) by means of the EGPRS PACKET CHANNEL REQUEST message, as shown at 21. In this way, the BSS(new cell) knows that the MS supports the EGPRS. The BSS(new cell) can then allocate a UL TBF in the EGPRS mode, as shown at 22, by sending a PACKET UPLINK ASSIGNMENT message, and can then resume the transfer in the downlink direction in the EGPRS mode.
The MS then forwards the TCP ACK with the number “n+2” to the BSS(new cell), as shown at 23. The BSS(new cell) forwards the TCP ACK to the SGSN, as shown at 24, and this serves as a cell update for the SGSN. As shown at 25, the SGSN then sends a FLUSH-LL message that commands the BSS(old cell) to reroute to the BSS(new cell) the LCC PDU(s) not yet transmitted in the downlink direction. The BSS(old cell) then sends a FLUSH-LL ACK message to the SGSN, as shown at 26.
To resume the transfer in the downlink direction, the BSS(new cell) then sends the MS a PACKET DOWNLINK ASSIGNMENT message, as shown at 27, advising the MS of the packet mode resources allocated to it, in this instance in the EGPRS mode. Transfer between the MS and the BSS(new cell) can then be resumed in the new cell, as shown at 28, where a TCP segment having the sequence number “n+2” is transmitted in the downlink direction and a TCP ACK having the number “n+3” is transmitted in the uplink direction.
A second scenario corresponds to the situation in which the MS has no LLC PDUs to send.
This second scenario corresponds to the example shown in FIG. 3. In this example, before going to the state 2′, TCP segments having sequence numbers “n” and “n+1” have been transmitted in the downlink direction and TCP ACKs having the numbers “n+1” and “n+2” have been transmitted in the uplink direction.
In this case, a shown at 21′, the MS requires a UL TBF to send a cell update message, and according to the current version of the standard, the MS can do this only by means of the CHANNEL REQUEST message (with the “one phase access” cause) or the PACKET CHANNEL REQUEST message (with the “cell update” cause). Unfortunately, the network does not know that the mobile station supports the EGPRS, which means that the network has no choice but to allocate a UL TBF in the GPRS mode, as shown at 22′, by sending a PACKET UPLINK ASSIGNMENT message. The MS then sends a cell update message to the BSS(new cell) as shown at 23′ (in fact this is an empty LLC PDU). The SGSN forwards the cell update message to the BSS(new cell), as shown at 24′. As shown at 25′, the SGSN then sends a FLUSH-LL message that commands the BSS(old cell) to reroute LLC PDU(s) not yet transmitted in the downlink direction to the BSS(new cell). The BSS(old cell) then sends a FLUSH-LL ACK acknowledgement message to the SGSN, as shown at 26′. Thus in this second scenario it is possible to distinguish between two situations (note that two situations could also be distinguished in the first scenario, but this was of no consequence in relation to the statement of the problems).
In a first situation (corresponding to the FIG. 3 example) the network can establish the DL TBF (for resumption of the transfer in the downlink direction) on the PACCH of the UL TBF created to send the cell update message. For resumption of the transfer in the downlink direction, the BSS(new cell) then, as shown at 27′, sends the MS a PACKET DOWNLINK ASSIGNMENT message advising the MS of the packet mode resources allocated to it, in this instance in the GPRS mode. The transfer can then be resumed, as shown at 28′, where a TCP segment having the sequence number “n+2” is transmitted in the downlink direction.
In this first situation, it will therefore be necessary to change the mode for the TBF afterwards, and the only way to do this is to release the UL TBF, release the DL TBF, and then re-establish a DL TBF in the EGPRS mode. The releasing of the UL TBF is illustrated by a state denoted 29′. As shown at 30′, during the state 29′, the BSS(new cell) sends the MS a PACKET UPLINK ACK/NACK message including in particular a final ACK indicator (FAI) bit equal to 1. As shown by a state 31′, the DL TBF is still operative. As shown at 32′, during the state 31′, a TCP segment having the sequence number “n+3” is sent in the downlink direction, after which the mobile station sends a PACKET CONTROL ACK message to the new cell, as shown at 33′. The releasing of the DL TBF and then the re-establishing of a DL TBF in the EGPRS mode are shown by a state denoted 34′. During the state 34′, an RLC data block including a final block indicator (FBI) equal to 1 is sent in the downlink direction, as shown at 35′, after which a PACKET DOWNLINK ACK/NACK message including a final ACK indicator (FAI) bit equal to 1 is sent in the uplink direction, as shown at 36′. Once the DL TBF in the GPRS mode has been released, a PACKET DOWNLINK ASSIGNMENT message can then be sent to the mobile station on the PACCH, as shown at 37′, this message indicating the packet mode resources allocated to the mobile station in the downlink direction, in this instance in the EGPRS mode. The transfer in the downlink direction is then continued in the EGPRS mode, as shown at 38′, and a TCP segment having a sequence number “n+4” is sent in the downlink direction and a TCP ACK having the number “n+3” is sent in the uplink direction.
We have found that the above kind of method is not the optimum since the GPRS mode is used instead of the EGPRS mode some of the time, and furthermore because the change from the GPRS mode to the EGPRS mode wastes time. To be more precise, if T denotes the time necessary to resume the transfer in the downlink direction in the EGPRS mode in the first scenario, in which the mobile station still has one or more LLC PDU(s) to send, and T′ denotes the time necessary to resume the transfer in the downlink direction in the EGPRS mode in the second scenario, in which the mobile station has no LLC PDU(s) to send, the time T′ can be expressed as follows:T′=T+T1+T2+T3+T4where:                T1 is the time necessary to be sure that the DL TBF has been established successfully,        T2 is the time necessary to release the UL TBF,        T3 is the average time necessary to transfer half of an LLC PDU on the TBF, and        T4 is the time necessary to release the DL TBF        with:                    T1=RTD+RRBP (where RTD is the round trip time between the BSS and the MS and RRBP is the time needed between an invitation to send, sent by the network, and the response of the mobile station),            T2=RTD+RRBP,            T3=½T_llc_pdu_transfer (with, for example, T_llc_pdu_transfer=200 ms in the case of a bit rate of 2.5 kbit/s and an LLC PDU size of 500 bytes), and            T4=RTD+RRBP.                        
Considering typical values RTD=120 ms and RRBP=60 ms, this can in some cases mean that the GPRS mode continues, in place of the EGPRS mode, for at least 60 ms.
In a second situation in the second scenario, not specifically shown in the figures, the network cannot establish the DL TBF on the PACCH of the UL TBF created to send the cell update message but can only establish the DL TBF on the (P)CCCH, and consequently after the release of the UL TBF created for sending the cell update message. In this case, the BSS(new cell) can directly allocate packet mode resources in the EGPRS mode. The BSS(new cell) then knows the capacities of the MS, thanks to corresponding information contained in the BSSGP frames received from the SGSN. However, the method is still not optimum because of the time wasted waiting for the release of the UL TBF before being able to establish the DL TBF on the (P)CCCH.
To summarize, we have become aware that problems arise due to the various scenarios for resuming the transfer in the downlink direction, according to whether the mobile station has LLC PDU(s) in its buffer or not. This leads to incoherent behavior in the case of cell reselection, especially since in the application based on the TCP the two situations can occur (the mobile station has LLC PDU(s) in its buffer or does not have LLC PDU(s) in its buffer), and moreover, as explained above, transfer in the EGPRS mode is resumed with a time-delay in the situation where the mobile station has no LLC PDUs in its buffer.
The example more specifically described hereinabove is the situation of a cell update in the case of cell reselection in the packet transfer mode. Similar problems arise in other examples, in particular the example of packet mode paging. With the current version of the standard, when the network sends a paging request to a mobile station for services in packet mode, the mobile station must respond by means of a CHANNEL REQUEST message (with a cause corresponding to a one phase access) or a PACKET CHANNEL REQUEST message (with a cause corresponding to a paging request response); thus the network will not know if the mobile station supports the EGPRS and will establish a UL TBF in the GPRS mode, even for a mobile station supporting the EGPRS. When the SGSN receives the response from the mobile station, it can start to send LLC PDUs to the correct cell, i.e. a transfer of user data in the downlink correction can then begin, but problems similar to those described above for the cell update situation therefore also arise, since if the UL TBF is still operative, then the DL TBF will initially be established in the GPRS mode.
As we have realized, the problems previously discussed relate generally to any scenario leading a mobile station to require a UL TBF to transfer signaling data, thereafter generating the establishment of a DL TBF for the transfer of user data.